The idea behind this possibly being a fire hazard is all heat.  Basically, the powerbricks you're hiding in the box generate heat as they turn normal household electric current into current usable by whatever it was you plugged in.  Now I'm not an electrician or an engineer of any kind, but I'm not convinced the powerbricks could generate enough heat to actually cause a fire in a box like this.  Still if you're really wanting to try this and really concerned about it possibly starting a fire; why not just had more holes for ventilation or replace one panel of the box with some kind of mesh or screen to let air in?